A brushless direct-current (BLDC) motor may be classified into a core type (or a radial type) and a coreless type (or an axial type), each having a generally cup-shaped (cylindrical) structure, according to whether or not a stator core exists.
A BLDC motor of a core type structure may be classified into an inner magnet type including a cylindrical stator where coils are wound on a number of protrusions formed on the inner circumferential portion thereof in order to form an electronic magnet structure, and a rotor formed of a cylindrical permanent magnet, and an outer magnet type including a stator where coils are wound up and down on a number of protrusions formed on the outer circumferential portion thereof, and a rotor formed of a cylindrical permanent magnet on the outer portion of which multiple poles are magnetized.
In a conventional outer magnet type BLDC motor, a main path of a magnetic flux forms a magnetic circuit starting from a permanent magnet of a rotor and proceeding toward a stator via an air gap, and proceeding toward the permanent magnet again and in the direction of a yoke.
In a conventional inner magnet type BLDC motor, a plurality of T-shaped core portions on a stator core around which coils are wound, protrude inwards. Also, the inner side ends of the respective core portions form a circle of a predetermined diameter. Also, a rotor is mounted in an inner space of the inner magnet type BLDC motor in which a cylindrical permanent magnet including a rotational shaft is attached, or at the center of the inner magnet type BLDC motor in which a ring-shaped permanent magnet is attached to a cylindrical yoke including a rotational shaft. The inner magnet type BLDC motor rotates in the same manner as that of the outer magnet type BLDC motor.
The magnetic circuit in the above-described core type BLDC motor has a symmetrical structure in the radial direction around the rotational shaft. Accordingly, the core type BLDC motor has less axial vibration noise, and is appropriate for low-speed rotation. Also, since a portion occupied by an air gap with respect to the direction of the magnetic path is extremely small, a high magnetic flux density may be obtained even if a low performance magnet is used or the number of magnets is reduced. As a result, a big torque and a high efficiency may be obtained.
The present applicant proposed a BLDC motor having a single-stator/double-rotor structure in a radial core type in which a stator core may be configured in a full split type, in Korean Patent Laid-open Publication No. 2004-2349. In the Korean Patent Laid-open Publication No. 2004-2349, rotors are respectively disposed at the inner and outer sides of the stator core, to thus form a flow of a magnetic path by a permanent magnet and a yoke that are respectively placed at the inner and outer sides of the stator core. Accordingly, the stator core may be perfectly split, to thus greatly enhance productivity and output of the motor by the individual coil windings.
In addition, in the case of motors having a single-stator/double-rotor structure respectively disclosed in Korean Patent Laid-open Publication Nos. 2008-30667 and 2008-666, coils are toroidally wound on a yoke placed between inner and outer slots formed in an identical number, and thus a high fill factor may not be available, to accordingly limit an efficiency rise.
Meanwhile, a conventional split-core/double-rotor type motor may be illustrated as shown in FIG. 1. FIG. 1 is a cross-sectional view of a conventional split-core/double-rotor type motor.
In the conventional split-core/double-rotor type motor illustrated in FIG. 1, a split type stator core (that is, a split-core) 1 is located between an inner rotor 2 and an outer rotor 3. The split-core/double-rotor type motor forms a magnetic circuit L0 in which the split type stator core 1 is opposed to the inner rotor 2 and the outer rotor 3 through a preset magnetic gap, respectively. Here, the magnetic circuit L0 forms a single path in which a magnetic flux passes through the split type stator core 1, the inner rotor 2, the split type stator core 1, and the outer rotor 3.
In general, the longer the path of the magnetic circuit may be, magnetoresistance may increase, and thus the loss of a magnetomotive force may become great. Accordingly, the conventional split-core/double-rotor type motor forms the magnetic circuit L0 in which a magnetic flux circulates between the inner rotor 2 and the outer rotor 3 with the split type stator core interposed between the inner rotor 2 and the outer rotor 3. Therefore, when compared with a single-rotor type structure, the conventional split-core/double-rotor type motor has large magnetoresistance to thus cause loss of a magnetomotive force and accordingly have an inefficient structure.
Thus, the conventional split-core/double-rotor type motor needs to reduce magnetoresistance by reducing the path of the magnetic circuit, to prevent the loss of the magnetomotive force.
Meanwhile, the Korean Patent Laid-Open Publication No. 2008-30667 disclosed a motor of a single-stator/double-rotor structure, in which a stator around the inwardly and outwardly protruding serrated saw-teeth of which coils are wound is combined with a double-rotor, to thus reduce cogging torque and torque ripples.